Answer
$-5x^4-4xy-9y^3$ has a degree of 4.
Work Step by Step
$$(x^4-7xy-5y^3)-(6x^4-3xy+4y^3)$$ $$=(x^4-7xy-5y^3)+(-6x^4+3xy-4y^3)$$ $$=(x^4-6x^4)+(-7xy+3xy)+(-5y^3-4y^3)$$ $$=-5x^4-4xy-9y^3$$
Because the term with the largest degree is -5x^4, the degree of the expression is 4.
(Add the degrees of each variable in the term to determine its degree.)
